Lithium-ion secondary batteries are common in portable consumer electronics because of their high energy-to-weight ratios, lack of memory effect, and slow self-discharge when not in use. Rechargeable lithium-ion batteries are also being designed and made for use in automotive applications to provide energy for electric motors to drive vehicle wheels.
Lithium-ion batteries may be formed in different sizes and shapes but three common functional components are the anode, cathode, and electrolyte that make up cells of the battery. The anode on discharge becomes the cathode on charge, and the cathode on discharge becomes the anode on charge. From here forward, we shall refer to the electrode that is the anode on discharge (the negative electrode) as the anode, and the electrode that is the cathode on discharge (the positive electrode) as the cathode. A porous separator is often used to contain electrolyte and prevent physical contact (electron-conducting contact) between the anode and cathode. Many cells may be arranged in series or parallel electrical current flow connection, or any suitable combination thereof, to meet the electrical potential and power requirements of a battery design.
A lithium-ion battery generally operates by reversibly passing lithium ions between a negative electrode and a positive electrode. The negative and positive electrodes are situated on opposite sides of a microporous polymer separator that is soaked with an electrolyte solution suitable for conducting lithium ions. Each of the negative and positive electrodes is also carried on or connected to a metallic current collector (typically copper for the anode and aluminum for the cathode). During battery usage, the current collectors associated with the two electrodes are connected by a controllable and interruptible external circuit that allows an electron current to pass between the electrodes to electrically balance the related transport of lithium ions through each cell. Many different materials may be used to produce these various components of a lithium-ion battery. But in general, the negative electrode typically includes a lithium insertion material or alloy host material, the positive electrode typically includes a lithium-containing active material that can store lithium at higher potential (relative to a lithium metal reference electrode) than the host material of the negative electrode, and the electrolyte solution typically contains one or more lithium salts dissolved and ionized in a non-aqueous solvent. The contact of the anode and cathode materials with the electrolyte results in an electrical potential between the electrodes and, when an electron current is exploited in an external circuit between the electrodes, the potential is sustained by electrochemical reactions within the cells of the battery.
A lithium-ion battery, or a plurality of lithium-ion batteries that are connected in a series or a parallel arrangement (or any suitable combination thereof) for current flow, can be utilized to reversibly supply power to an associated load device. The battery system delivers electrical power on demand to a load device such as an electric motor until the lithium content of the negative electrode (anode) has been depleted to a predetermined level. The battery may then be re-charged by passing a suitable direct electrical current in the opposite direction between the electrodes.
At the beginning of the discharge, the negative electrode of a lithium-ion battery contains a high concentration of intercalated lithium while the positive electrode is relatively depleted. The establishment of a closed external circuit between the negative and positive electrodes under such circumstances causes the transport of intercalated lithium from the negative anode. The intercalated lithium is oxidized into lithium ions and electrons. The lithium ions are carried from the negative electrode (anode) to the positive electrode (cathode) through the ionically conductive electrolyte solution contained in the pores of the interposed polymer separator while, at the same time, the released electrons are transmitted through the external circuit from the negative electrode to the positive electrode (with the help of the current collectors), to balance the overall reaction occurring in the electrochemical cell. The lithium ions are assimilated into the cathode material by an electrochemical reduction reaction. The flow of electrons through the external circuit can power a load device until the level of intercalated lithium in the negative electrode falls below a workable level or the need for power ceases.
The lithium-ion battery may be recharged after a partial or full discharge of its available capacity. To charge or re-power the lithium-ion battery, an external power source is connected to the positive and the negative electrodes to drive the reverse of battery discharge electrochemical reactions. That is, during charging, the lithium within the positive electrode is oxidized to yield lithium cations and electrons. The cations transport across the separator to the negative electrode, and the electrons travel through the external circuit to the negative electrode as well. At the surface of the negative electrode material, the lithium cations are reduced to lithium by combining with the available electrons within the negative, and the negative electrode lithium content increases. Overall, the charging process reduces the lithium content within the positive and increases the lithium content within the negative.
The separator serves an important function in each cell of a lithium-ion battery. In many lithium-ion battery constructions the negative and positive electrode materials are formed as thin, compacted, polymer bonded, particulate material layers on their respective current collectors (for example, copper or aluminum foils) and each cell is assembled with a thin, porous, polyolefin separator membrane inserted between the facing electrode layers. For example, polyethylene or polypropylene fibrous membranes have been used having a thickness of about twenty-five to about thirty microns and thirty-five percent or more porosity. Very small, open pores are formed in the thin polymer sheet to permit a liquid electrolyte to enter and flow through the separator membrane.
Thus, the pores and surfaces of the polyolefin membrane are filled and contacted with a lithium ion-containing, non-aqueous electrolyte that contacts and wets the facing electrode materials to enable the flow of lithium ions and counter-ions through the pores of the separator and between the electrodes. But the polymeric membrane resists the flow of electrons directly between the electrode materials.
Prior polymer separators have been filled with small particles of a ceramic material, such as silica or alumina, or surface-coated with polymer-bonded particles of such ceramics. The purpose of the ceramic particle additives has been to increase the puncture strength, dimensional stability at high temperatures (above which polymers such as polyethylene or polypropylene would exist in a molten state), and electrolyte retention capability of the separator membrane. However, the inventors herein have sought and found an improved method of preparing surface-coated porous separator structures and materials.